The amount of time we spend sitting predicts our risk for gaining weight, even if we do go to the gym (or run or fence or whatever) at some point in the day. Taking frequent breaks from being sedentary, even to do something that requires very minimal effort—and/or sitting less altogether—has been proven to be significantly beneficial to metabolism.
I like to think of exercise like diet. One salad a day is good, and so is one workout. Ideally, though, you're always eating moderately well and always being decently active. Health initiatives are getting away from the "Do vigorous exercise for 20 minutes and it’s okay if you sit around for the other 23.66 hours" mantras and toward "Physically do things." When I am Secretary of Health and Human Services, that will be on every bus. I'll see if I can get it printed on the currency. I don't know how things get on the currency.
Personally I'm not so motivated by preventing metabolic disease in my quest for office exercise as I am by just feeling alive, which sitting for ten hours does not make me feel.
Sitting is ingrained in office culture, though. How do you get around it? Office workers have begun clamoring for standing desks (and bicycle desks) of late, but they still aren't the norm. In many corners, too, incorporating physical activity into an office workday isn't yet "cool." How does it become cool? I don't know, but here's what I do.